A brief survey is presented of areas of progress on traveling wave tube designs. Data demonstrates the effect of multistage depressed collectors, the design of which is made possible by powerful NASA computer programs. Other topics include beam refocusing, RF circuit losses, and cathode testing

Buck, E.

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https://ntrs.nasa.gov/search.jsp?R=19780011454 2020-03-22T06:01:50+00:00Z
NASA TECHNICAL	 NASA TM-78852
MEMORANDUM
(NASA-Tm-78852)	 TIP-D 1TE OF TRAVELING WAVE	 N78-193117
TITBE I^"PF.OVFMSNTS	 15 F HC A02/`iF A01
CSCL )qr
Unclas
6.3/33	 08684
U P-DATE OF TRAVELING WAVE TUBE IMPROVEMENTS
by Erik Buck	 I
Lewis Pesearch Center
Cleveland, Ohio 44135	 1
TECHNICAL PAPER to be presented at the
Electronic Warfare Symposium
Warner Robins Air Force Base, Georgia, March 20-24, 1978
1;►AR 19713
UP-DATE OF TRAVELING WAVE TUBE IMPROVEMENTS
by Erik Buck*
National Aeronautics and Space Administration
Lewis Research Center
Cleveland, Ohio 44135
This paper is a brief survey of areas of progress on traveling wave tube
designs, with emphasis on recent work by NASA Lewis Research Center. Data
demonstrates the effect of multistage depressed collectors, the design of which
is made possible by powerful NASA computer programs. Other topics include
beam refocusing, rf circuit losses, and cathode testing.
ti
a:
	 INTRODUCTION
Because of the importance of traveling wave tubes (TWTs) for space com-
munications, NASA Lewis Research Center has for several years investigated
the physics of electron beam devices. Much of the technology is directly ap-
plicable to the TWTs used in electronic warfare systems. There exist agree-
ments between NASA, the Air Force, and the Navy to cooperate in the transfer
of NASA-developed technology to the TWT industry. This paper is a brief sur-
vey of recent NASA activities which could benefit the electronic warfare com-
munity.
THE SIGNIFICANCE OF TWTs
Figure 1 depicts the prime power budget of a typical electronic warfare
subsystem. Nearly two-thirds of the power is constuned by the power amplifier
which, typically ; is a traveling wave tube. Of the power consumed by the ampli-
fier, perhaps four-fifths is wasted and appears as heat which must be removed
by a cooling system. If the efficiency of the TWT amplifier were doubled, the
r
	 prune power requirement could be reduced by a third, and the critical thermal
load would be halved.
*Major, L' . S. Air Force; Laboratory Associate NASA Lewis Research
Center, Cleveland, Uhio.
STAR Category 33
Tf
2
TWT COMPONENTS
Figure 2 is a schematic of a typical TWT, with the gun structure (greatly
enlarged) on the left and the collector on the right. The gun forms an electron
beam which travels from left to right and impacts on the collector. The beam
is confined by a magnetic field and enclosed by a slow wave structure which
forms the rf circuit of the tube. ECb1 tubes nearly always use a helix-
structure rf circuit to obtain a wide bandwidth. At the input section of the rf
circuit, the interaction between the traveling rf wave and the electron beam
modulates the beam, causing the electrons to bunch. Toward the output, the
bunched electrons hiteract with the rf wave, causing it to be amplified. The
interaction converts some of the kinctic energy of the electrons to rf electro-
magnetic energy, and the greater the interaction, the more disorderly the
electron beam becomes. The spent electron beam emerges from the output
section of the helix and enters the collector with a range of velocities. In the
past, collectors operated at or near ground potential, and all of the residual
kinetic energy of the electrons was converted into heat.
MULTISTAGL DEPRESSED COLLECTORS
The collector shown in figure 2 is a simple two-stage depressed collector,
representative of the latest production tubes. It is multistage in that it oper-
ates at two different potentials, and it is depressed in that the collector stages
are negative relative to ground. The ohject of this arrangement is to sort the
electrons into velocity groups and to recover most of their kinetic energy by
letting them do work against the negative potential. This requires the two addi-
tional power supplies, E C1 and E C ,) , in the lower right corner of figure 2.
The benefits of a multistage depressed collector (MDC) are shown in fig-
ure 3. It is evident that a tube with a low electronic efficiency, 0.1 or 0.2 of
the beam power converted to rf, can be close to 50 percent efficient overall if
most of the energy of the spent beam can be recovered and other losses are
small. A good MDC can multiply tube efficiency by two or more.
Analysis of the Sterzer collector [ 1] , figure 4, showed that because such
a collector acted as a convergent electron lens, :pace charge blocking occurred
and the collector cc,uld not be depressed very far. Depressed collectors were
widely thought to be inherently limited and impractical Ilowever, if a more
sophisticated approach is used and electron trajectories are properly computed,
allowing for the effects of electrostatic lenses and Lhe space charge, it is pos-
sible to build practical collectors of greater than .S0 prreent efficiency. One
I'M
A
i
1
3
such collector has been operating for 2 years aboard the Communications
Technology Satellite.
Table 1 demonstrates that the performance of such collectors may be pre-
dictable; in this case the compiled currents agree with measurements oil
experimental collector. (The collector programs are still being improved,
debugged, and verified against experimental data.) As with the simple sche-
matic collector of figure 2, two additional power supply voltages must be pro-
vided, but the pay-off is that more than 80 p, reent of the spent beam energy is
recovered, reducing the power, and the total weight of power supplies, needed.
Figure 5 is a computer-generated depiction of electron trajectories in the
collector described in table 1. The drawing is half of an axisymmetric longi-
tudinal section. The lines which go from top to bottom are equipotential lines
which show the complexity of the electrostatic lenses formed by the plates of
the collector. The net effect is of a divergent lens which drives the electrons
toward walls and prevents them, or secondary electrons generated by impacts
oil 	 walls, from back st ream ing into the tube.
It is not true that inevitably as collector depression increases, beam inter-
ception oil 	 helix increases. It would be m-.)re correct to say that poorly de-
signed collectors fail to trap low energy electrons, so that, as the depression
is increased, more of the electrons find their way back into the rf section of
the tube, going in the wrong direction. The effect is power loss (and possible
rf instability) which is often attributed to the beam being intercepted by the rf
circuit.
This NASA collector design has been adapted by at least two manufacturers
and is being applied to an experimental tube to be delivered to the Air Force.
However, the design, as shown, is larger and more comp; ,_­: than it need be.
Figure G is a more recent iteration of the collector design.. It has not yet been
built, but the computations indicate shat it is as efficient a., he former collec-
tor, easier to make, and less than 0.03 meter (about an inch) ill diameter. Note
that must of the electron: impact oil 	 walls, so that the path for heat conduc-
tion can be short.
The NASA collectors are preceded by a short refocusing section between
the rf circuit and the collector proper, as shown in figure 7. The effect is to
expand and collimate the electron beam, so that the electron velocity vector is
more nearly parallel to the collector axis. The object is to make it easier to
sort the electrons by energy and to reduce the number of electrons which might
be reflected back into the tube.
4THE NASA 3-D HELICAL COMPUTER PROGRAM
Tilt- design of an efficient collector depends upon a knowledge of the three-
dimt • nSional trajectories of the electrons as they pass from the rf output section
through the diminishing fields of the refocusing section into the collector. As
one cannot directly measure the velocity of individual electrons, the Power
Amplifier Section at NASA Lewis turned to a computer simulation. Table 2
Fsts the inputs and outputs of the NASA program, which is probably the most
Powcc•ful progr:1111 available to the tube designer. To compute JG electron tra-
jectories for a t^pic•al octave- bandw idth tube requirCS about 30 minutes of Cen-
tral processing unit time oil 	 1140 Unicac computer. The program source
deck is available from NASA to boi;a fide users.
'fable .3 indicates that the computer model does, ill
	 closely approxi-
mate the belia ,.ior of an actual tuiv. There is need for more cases to validate
the model, but there is a problem ill
	 the precision of the comPutcr model
exceeds that of the input data ill
	 case;. Une must have accurate mcasure-
ments of a IxIrticular single tube, since often the variability of supposedly iden-
tical tubes, from the same production line, is greater than the error in the
model.
It is probable that the NASA program accurately models all the significant
variables in tube performance. Within the near future, a ^rl Lori tube design
Nvill be realizable. Instead of the present practice of builduig many cut-and-try
experimental tubes, it should be Possible to do it right the first time.
M
I	 .
i
d
.	 I
RF CIRCUIT LOSSES
As long as TWTs used inefficient collectors, the losses ill
	 rf circuit
were relatively inconspieuouS. As figure N shows, with an efficient collector,
the rf losses become significant. Some rf energy is lost in the dielectric sup-
ports of the hcli\; other losses are causedby the electrical rcSiSLanCe of the
helix and of the Nvalls of the tube. The LN-Pic:11 tube flying today has a tungsten
helix. 1'he tube dcsignoi • mat . have estimated the rf losses as 5 or lU percent
of the rf power output. In tact, the losses may be 25 Percent. The surface of 	 11
the hell\, which lucks well. Polished, probably is covered with microscopic Pits
and cracks which increase losses at high frequencies. As the helix gets hot,
the resistance of tht' metal incrcaSCS, so the helix gets (totter still.
Designers of more efficient tubes, unless they ( , .III a better ti':I%- to
fabricaLe tungsten, will pruhahl^\ be forced to use a more conductive metal,
copper. A soft copper helix cannot be run hot. It requires good ColldUetivl•
cooling and inet-lt:inie:il Support. There will probabl y lie a trend LOWard Cuppc[•
1
5or copper plated circuit parts braze ,4
 *.o support structures for thermal conduc-
tivity and mechanical strength.
The rf output coupler should not be overlooked. In a broadband tube it is
very difficult to get all high-power rf signals from the end of the slow wave
structure, through a vacuum-tight window, and into a wave guide without ab-
sorbtion or reflections. Such losses cause local heating at the output which can
destroy a tube.
There is a strong incentive for tube builders to continue the trend toward
periodic permanent magnet (ppm) focusing of the beam. The "easy way" is to
use a solenoid, but solenoids are big and heavy. They use power, perhaps a
kilowatt, and generate heat which must be removed. Perhaps we will see
hybrid tubes with some permanent magnets and a smaller solenoid. Computer-
aided design will help to achieve better control of the beam and thus to minimize
interception of the beam by the rf circuit.
CATHODES AND GUNS
The gu.n which forms the electron beam is clearly a critical component.
NASA is investigating the physics of gwns, particularly the mechanism of
cathode emission. Table 4 reports the status, ill 	 of a long-term study
of cathode life. Some conclusions are summarized in table 5. Note that th,^
cold emission cathode, which is being investigated for NASA by Stanford Re-
search Institute, offers significantly higher current density.
A cold cathode is pictured in figure 9. Conventional cathodes fail when
the emissive surface is depleted or poisoned. In theory, the cold cathode
might last forever. In practice, it fails by catastrophic accident, when there
is arcing between the paints and the gate film. Arcing call
	 if there is a
gas near the cathode or it there are excessive transient currents during pulsed
operation. If cold cathodes are employed in tubes, the gwn will be entirely
redesigned.
CONCLUDING liEAIARKS
The Power Amplifier Section at NASA Lewis Research Center has several
projects which Sh(juld benefit the tube bidustiy in general and the electronic
warfare commw1ity in particular. NASA is investigating; the basic physics of
guns and collectors and is perfecting a computer model of the interaction be-
tween the electron beam and the rf circuit. These studies may be very useful
to tune industry designers as they push traveling wave tube efficiencies toward
ORIGINAL PAGE, I^
OF POOR QUALITY
G• 50 percent. Military program managers, too, should monitor these develop-
ments and appreciate the gains in system performance which can be realized
by cooperative efforts to disseminate new techniques.
REFERENCE
1. Sterzer, F.: Improvement of Traveling-Wave-Tube Efficiency through
Collector Potential Depression. IRE Electron Devices. ED5, no. 4,
Oct. 1958, pp. 300-305.
1.
T' 0
Ii
-i
TABLE 1. - GUNS AND BEAMS EXPERIMENTAL AND
	
I
	 ANALYTICAL* MDS PERFORMANCE COMPARISONf
(AT SATURATION NEAR MID-BAND WITH 2-STAGE COLLECTOR)
COLLECTOR	 MEASURED	 COMPUTED
VOLTAGE,	 COLLECTOR
	 COLLECTOR
V	 CURRENT,
	 CURRENT,
	
mA	 mA
LOLL (1)
	
0	 26	 27
COLL (2)	 5230
	 73	 80
COLL (3)
	 5230	 149
	 134
COLL (4)
	 8680
	 13	 13
COLL (5)
	 8690	 158	 154
COLL (6)
	 9440	 14	 20
MEASURED	 COMPUTED
COLLECTOR EFFICIENCY
	 82.1%	 83.2%
	
'	 POWER DISSIPATED IN
	 595 W	 630 W
COLLECTOR
"INITIAL CONDITIONS DERIVED FROM TWT AND REFOCUSING
SYSTEM TRAJECTORY CALCULATIONS.
TABLE 2. - NASA LeRC 3-D HELICAL COMPUTER PROGRAM.
INPUTS:
BEAM RADIUS, CURRENT, VOLTAGE AT ENTRANCE TO HELIX
MAGNETIC FIELD DATA, SOLENOID OR PPM
CAN BE UP TO FOUR COSINE TERMS FOR EACH SECTION OF RF CIRCUIT
DIMENSIONS OF HELIX STRUCTURES, INCLUDING TAPERS AND UP TO THREE SEVERS
IMPEDANCE, PHASE VELOCITY, AND LOSSES IN dB PER INCH FOR EACH SECTION
OUTPUTS:
RF POWER, GAIN
CIRCUIT LOSSES
INTERCEPTION, AS PERCENT OF CURRENT, PERCENT OF POWER
AXIAL, RADIAL, AND ANGULAR VELOCITY COMPONENTS FOR 32 OR 96 CENTROIDS
OF CHARGE
BACKWARD WAVES ARE TREATED AS PERTURBATIONS
HARMONIC POWER CAN BE ADDED
I
f
r.aY	 s.. ti. c. _ 7 z,
ii
BASIC TWT PARAMETERS
MAGNETIC FIELD
CATHODE FLUX
IMPEDANCE
PHASE VELOCITY
GEOMETRY OF TUBE
LOSS
VO LTA GE
CURRENT
HELIX DATA, PITCH, RADIUS
POWER INPUT
ATTENUATOR DATA
CATHODE GEOMETRY
TABLE 3.
LeRC 3-D HELICAL PROGRAM
MAGNETIC PROFILE PROGRAMMED
AT LOW EDGE
	
2.5 GHz
GAIN
	
27.2dB
POUT
	
1700 W
INTERCEPTION
	
0
AT MIDBAND
	
3.25 GHz
GAIN
	
28.1 dB
POUT
	
2 048 W
INTERCEPTION
	
0
U.S. NAVY CRANE
MEASURED
PERFORMANCE
V. 3 dB
1715 W
20mA•1.5%
28.0
2 000 W
1.1%
TABLE 4. - LONG LIFE HIGH CURRENT DENSIT Y CATHODES
CONTRACT NAS3-14385 WITH WATKINS-JOHNSON
(STARTED 6(29171)
OBJECTIVE TO COMPETITIVELY EVALUATE THE PERFORMANCE OF THREE CATHODE TYPES
BY ENDURANCE TESTING.
APPROACH: 1. DESIGN GUN AND TUBE FOR 2 AMPSICM 2 CATHODE CURRENT OPERATION.
2. DESIGN TEST RIGS FOR AUTOMATED LIFE TEST.
3. EXPERIMENTALLY EVALUATE PERFORMANCE AS A FUNCTION OF LIFE FOR
4 CATHODE TYPES SHOWN BELOW:
CATHODE TYPE	 CURRENT	 OPERATING	 LIFE RESULTS
DENSITY	 TEMPERATURE
PHILIP'S	 2 Alcm 2	1100° C	 25 000
IMPREGNATED
SEMICON DISPENSER 	 2 Alcm2
G. E.: TUNGSTATE	 2 AIcm2
M-CATHODES	 2 Alcm2
1050 C 23 000 TERMINATED
9250 C 5 000 TERMINATED
110()° C 15 000
TYPE
OXIDE
PHILIPS "B"
PHILIPS "M"
PHILIPS "M"
I	 Cam' r) EM. CATH
TABLE 5. - CATHODES - TRADE-OFF.
CURRENT DENSITY 	 TEMPERATURE
0. 200 Alcm 2 800° C
2 AIcm 2 11000 C
2 Alc m 2 10000 C
0. 5 AIcm 2 9000 C
10 Alcm 2 200 C
EXP. LIFE
	
ORIGIriAl,pAGP, IS
50 000 hr	 or Poult QUAury
30 000 hr
50 000 hr
100 000 hr
1.11
i l n
7 ----.t—
12% 1 \	 50%
POWER	 LOW LEVEL
SUPPLY	 CIRCUITRY
POWER
OUTPUT
ORIGINAL PAGE IS
OF PWIt QUAUTY
1
L
i
t
	 TOTAL SYSTEM PRIME POWER
FINAL AMPLIFIER
HEAT
Figure 1. - ECM system. Prime power budget.
i
I
I
INPUT POWER
GRID?
+	 ATTENUATORHEATERSHADOW GRID-,++ ANODE
	 MAGNET -
CATHODES +	 '
If	 ^^ ++
Ik	
Ig	 Ih
= EhT	 T
OUTPUT POWER
COLLECTOR 1
ROD -
,	 r- COLLECTOR 2
+
+
+
HELIX
IC1 A,	 IC2
EC1=	 =EC2
Figure 2. - Schematic of TWT and power supplies.
rI'
f
1.0
FOR 7I CT < 1	 71e -ELECTRONIC
'TCT''le	
nc . C
COLLECTOR
	
EFFICIENCY
° • 1 - 'I c "Te'	 nC OVERALL
NO INTERCEPTION
NO CIRCUIT LOSSES
NO SECOND HARMONIC POWER
0	 .2	 .4	 .6	 .8	 1.0
COLLECTOR EFFICIENCY, '►c
Figure 3. - Overall efficiency vs. collector efficiency.
8
0
F'
Y
z .6
W_
U_
^i
W
J
Q--r .4
Q'
W
0
2
TUBE V0-,\ BMW
ELECTRON
BEAM	 E1	
VO > V 1 >V2
all 
E2
FRONT	 MAGNETIC
COLLECTOR V 1 -'	 SHIELD
c, -7INSO	 F.2> E 1
 - CONVERGENT LENS
Figure 4. - Sterzer (RCA) collector.
'-BACK
COLLECTOR
V2
fir
DMINAI, i
cc POOR QLAI..
r"m
a C.
I	 'I	 I'	 I
V
, T— :71r- -- 1 LOTT "- 	 r
4
ol
Ll
`^^ cft.4el E
CL)
ca.
x
cu
O	 y	 J	 ^ ^	 .1 ^ ^1 ^i j
	 ^ a
	
t5
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...................
En
t/
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Ilk 11 t
i
LIN
/	
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i 1 ^1	 a V TV
	
I	 xa
w
A
z
v
>r
	,^J ••,•	 OJ'J.	 YJ
T1	
^	 1	 ^ -L 1
	
t	 I
4
IDRIGWAL PAGE IS
OF ppOIt QUA11'TY
1.1
IF?
	
E
4
.2
'?c t
1.0
0.8
).6
FED
L SUSCH'S THEOQLBA
Y"	
-IBZ
Z ADIABATIC BEAM EXpANSION
^.-PLATEAU
	 rl BZO
Ll —	 ^2 --T	 r0 n BZI
BZI 3. ELECTRON DIPOLE AWM f
Z	 P + ll WILY • CONST
TRArIEJCTORES
Ol1TfMR
I
rl
Ir0	 Z
Figure 7. - Schematic of refocusing system.
RADIAL VEL M RDtLIM
^a-krz 1
k zd
Gs-71K/
0	 .2	 .4	 .6	
.8	 1.0
nC
	Figure 8 .
	 Effect of cirruIt losses on the overall tube efficiency
for ne•0.15.
0.001 inch
25
Figure 9. - Field emission cathode.
Ptic
Q	 I's
UALFIi


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